Scale insects on Plants

Symptoms

Scale problems show up both on the plant and in the space underneath it. You may find wet or sticky honeydew on leaves, pavement, cars, or anything else beneath the infested plants. Black sooty mold can also grow on that honeydew, and it is a second sign worth looking for.

The plant itself may have discolored, distorted, or dying leaves, twigs, or branches. Check leaves, twigs, and branches, because the symptoms are listed for all three. Wet honeydew can land on cars and pavement, not only on leaves.

Look closely at the bark, leaves, and fruit as well. Adults may appear as circular, elongate, or oval bumps or discolorations on bark, leaves, or fruit. Those bumps can differ in shape, from round to long to oval, so not every one will look alike.

Taken together, the signs are sticky honeydew, black sooty mold, discolored or dying growth, and bumps on the plant. Notice that honeydew and mold appear on surfaces below the plant, while the bumps appear on the plant itself.

Causes

Populations of some scales can increase dramatically within a few months when the weather is warm. Warm weather is therefore one condition that lets numbers climb fast.

Honeydew-seeking ants protect scales from their natural enemies. Scales produce honeydew, and the ants that seek it out guard the scales. Natural enemies of scale, which include parasites and predators, are the insects that keep scale populations down.

Natural enemies can also be disrupted by dust, as well as by ants. Disruption of natural enemies is listed as a cause of scale problems, whether the ants or the dust is behind it. Ants matter because they protect scales from their natural enemies, so ant control appears in the advice for conserving those enemies.

In short, three conditions are recorded: warm weather, honeydew-seeking ants, and disruption of natural enemies by ants or dust. Conserving resident natural enemies is a listed treatment approach, which includes ant control and careful pesticide management.

Diagnosis

Start by deciding which signs you can actually see. Discolored, distorted, or dying leaves, twigs, or branches are one scale symptom, but the list has others. The other signs are sticky honeydew, black sooty mold, and circular, elongate, or oval bumps on bark, leaves, or fruit.

Wet or sticky honeydew on leaves or on surfaces below the plant points toward scale. Bumps or discolorations on bark, leaves, or fruit that are circular, elongate, or oval also point toward scale, since adults may appear that way. Sooty mold on the honeydew adds to the picture.

Ants moving about beneath the plant fit the causes, since honeydew-seeking ants protect scales from natural enemies. Warm weather also fits the picture, because scale numbers can rise dramatically within a few months when it is warm.

A plant with only discolored leaves and none of the other signs does not match the symptom list for scale as fully. Check the bark, leaves, and fruit for bumps before you decide.

Treatments

Provide plants with good growing conditions and proper cultural care, especially appropriate irrigation, so they can tolerate some scale feeding. You can also prune off heavily infested twigs and branches to physically remove scale populations. Consider replacing problem-prone plants that are repeatedly or severely infested.

The organic approach is to conserve resident natural enemies, which are parasites and predators of scale. You can do this through ant control, habitat manipulation, and careful pesticide management, rather than relying on insecticides. Growing a variety of flowering plants near susceptible plants can attract and support the natural enemies that keep scale populations down. These organic steps rely on natural enemies rather than on insecticides.

The chemical options in the advice are a dormant-season oil, a systemic insecticide applied in a certain way, and ant baits. A well-timed, thorough spray of horticultural (narrow-range) oil during the dormant season smothers scales. When a systemic insecticide is warranted, apply it to the soil or as a trunk spray rather than as a foliar spray.

Enclosed pesticide baits are insecticide mixed with an attractant. Place them near ant nests or on ant trails beneath plants to disrupt the ants that protect scales.

This page gives no dose, dilution, or spray interval for any product. The product label is the legal instruction, so follow it for the crop and site you are treating.

Prevention

Good growing conditions and proper cultural care, especially appropriate irrigation, let a plant tolerate some scale feeding. Keeping natural enemies active is another condition that matters, since they keep scale populations down. Ant control belongs here too, because honeydew-seeking ants protect scales from their natural enemies.

Dust can disrupt natural enemies, so dust is a condition to note. Careful pesticide management is part of conserving those enemies. Growing a variety of flowering plants near susceptible plants attracts and supports natural enemies.

Warm weather lets scale populations increase dramatically within a few months, so conditions in warm periods are worth watching. If a plant is repeatedly or severely infested, replacing it is an option.

Plants most often affected

• American Basswood
• Arabian Coffee
• Barbados Aloe
• Bird-of-paradise -tree
• Black Cherry
• Christplant
• Common Coleus
• Common Wormwood
• Dracaena masoniana
• Dwarf Umbrella Tree
• Eastern Redbud
• Edible Banana
• English Ivy
• European Mountain Ash
• Glossy Buckthorn
• Goldenrain Tree
• Inchplant
• Jade Plant
• Japanese Maple
• Littleleaf Linden
• Poinsettia
• Porcelainflower
• Pride of India
• Sugar Maple
• Sweet Cherry
• Weeping Fig
• Wild Hydrangea
• Yellow Trumpetbush